Data communication networks may include various computers, servers, nodes, routers, switches, bridges, hubs, proxies, and other network devices coupled together and configured to pass data to one another. These devices will be referred to herein as “network elements.” Data is communicated through the data communication network by passing protocol data units, such as data frames, packets, cells, or segments, between the network elements by utilizing one or more communication links. A particular protocol data unit may be handled by multiple network elements and cross multiple communication links as it travels between its source and its destination over the network.
The various network elements on the communication network communicate with each other using predefined sets of rules, referred to herein as protocols. Different protocols are used to govern different aspects of the communication, such as how signals should be formed for transmission between network elements, various aspects of what the protocol data units should look like, how packets should be handled or routed through the network by the network elements, and how information associated with routing information should be exchanged between the network elements. Networks that use different protocols operate differently and are considered to be different types of communication networks. A given communication network may use multiple protocols at different network layers to enable network elements to communicate with each other across the network.
One protocol, commonly referred to as MultiProtocol Label Switching (MPLS), specifies a way in which a label switched path may be established through a network. Briefly, label switch routers within the network are configured to read a label associated with a packet, replace (often referred to as swap) the original label with a new label, and forward the packet out of a corresponding port. Routers along a label switched path through the network will exchange labels in this manner to forward traffic along the path. To enable more than one customer to use a given path, a PseudoWire (PW) tag may be applied to the traffic at the ingress to allow the traffic to be differentiated at the egress. A service that utilizes a pseudowire will be referred to as a Virtual Private Wire Service (VPWS). Multiple protocols operate together to enable a MPLS network to function correctly. For example, there is a protocol that governs distribution of labels, another protocol that governs distribution of PW tags, a protocol that governs establishment of the label switched path, a routing protocol such as an Interior Gateway Protocol run between the network elements, and numerous other protocols as known to a person skilled in the art.
In operation, the MPLS network will establish label switched paths through the network using a Label Distribution Protocol (LDP). As part of this process, the LDP will allow the Label Edge Routers (LERs) to exchange Forwarding Equivalency Class (FEC) to label bindings to permit the LERs to map traffic to LSPs corresponding to the desired LER for egress from the MPLS network. When a frame arrives at the ingress LER, the LER will map the frame to a FEC, either by packet inspection or port/service association, and hence select a label to apply to the frame. The label will be used (swapped) by the Label Switch Routers (LSRs) to forward the frame across the Label Switched Path (LSP) through the network. Further nested labels such as PseudoWire labels may have been applied by the ingress LSR to permit the egress LER to identify the forwarder that should handle the frame at the egress, to cause it to be forwarded to the correct customer.
MPLS networks typically operate on top of a lower layer network such as a SONET/SDH network, Optical Transport Network (OTN), or layer 2 packet-switched technology such as Ethernet. Several types of Ethernet networks may be used to carry MPLS traffic, such as Ethernet defined by IEEE 802.1ah (Provider Backbone Bridging or “PBB”), IEEE 802.1Qay (Provider Backbone Bridging—Traffic Engineering or “PBB-TE”), or IEEE 802.1aq (Shortest Path Bridging, also known as Provider Link State Bridging).
Network operators are increasingly becoming interested in having the ability to perform fine-grained traffic engineering on MPLS networks. Simultaneously, network operators may seek to avoid tandem routing (transit of intermediate label switch routers on the LSP in which label-swapping is the only data path operation), by moving traffic off the IP/MPLS routed path, and onto a path at the lower layer, referred to herein as the “bypass layer”. This enables the MPLS traffic to bypass the tandem routers or permit them to be eliminated from the network entirely. As noted above, the lower layer is typically SONET/SDH, OTN, or a layer 2 packet-switched technology. An Ethernet handoff is often used between the IP/MPLS layer and the bypass layer.
To enable paths in the bypass layer to be used to move traffic off the LSP to avoid the tandem routers, the network is constructed such that the label edge routers are directly connected by the bypass technology. The term “bypass technology” will be used herein to refer to the lower layer network protocol implementing the bypass layer over which the MPLS network is running.
Internet Engineering Task Force (IETF) Request For Comments (RFC) 4206 discloses one technique by which the overhead of superfluous routing exchange may be avoided. In IETF RFC 4206 a Forwarding Adjacency (FA) is created administratively between the end-point routers which they may use to forward traffic between each other in preference to the normal routed path. Such a Forwarding Adjacency can include path segments installed in the underlying bypass technology only if the bypass technology is using the same Control Plane instance as the MPLS layer, which is not always desirable. However, if the desired adjacency appears as a point-to-point link at the routing layer (the client MPLS layer), typically because the bypass layer is running an autonomous Control Plane, then the adjacency will be advertised into the routing protocol in use on the MPLS network as if it were a physical link connecting the two end-point routers within the MPLS network. Advertising the bypass links as if they are physical links connecting pairs of end-point routers within the MPLS network will cause a separate Routing Adjacency to be established for each such link. This is because, in this model, the MPLS layer and the bypass layer have a client-server relationship, with the bypass layer offering complete transparency to the client layer and providing no visibility of the bypass (server) layer topology. At the lower layer, e.g. the Ethernet layer, traffic engineered paths may be used to convey the traffic flows between the end-point routers.
The RFC 4206 concept of administratively created Forwarding Adjacencies are both created and used as a TE link by exactly the same instance of the GMPLS control plane. Thus, the concept of a Forwarding Adjacency is applicable only when an LSP is both created and used as a TE link by exactly the same instance of the GMPLS control plane. Accordingly, if the Forwarding Adjacency is set up over a bypass connection using this method implied by RFC 4206 (using control plane peering rather than the client-server model) then the FA is not seen as a transparent connection at the routing layer, and the ports on the end-point routers which interface to the bypass technology must still form Routing & Signaling Adjacencies, except now with the bypass technology layer rather than the remote end-point routers. Thus, in this manner, the IGP in use at the MPLS layer would form a Routing Adjacency with the underlying technology rather than directly with the other end point MPLS router. This requirement for control plane peering between MPLS and the bypass technology is liable to be significantly more complex operationally than the traditional client-server relationship between MPLS and transport. Furthermore, this integrated control plane model is a poor fit to the internal structure of many communications providers, where separate organizations have total responsibility for the operation of different layers of the overall network.
As noted above, although it is possible to use the bypass technology to enable traffic to be forwarded directly between pairs of MPLS routers and away from the tandem routers, doing so frequently results in the creation of a large number of routing adjacencies due to the number of LERs directly connected at the MPLS layer. Since implementing Routing Adjacencies consumes a relatively large amount of processor resources on the MPLS routers, there is a practical limit to the number of Routing Adjacencies, and hence paths through the bypass layer, that may be implemented at a given MPLS router. Accordingly, it would be desirable to provide a way to reduce the number of routing adjacencies as only a small number are actually needed for robust exchange of routing information.